The person skilled in the art knows numerous all-optical clock recovery circuits, and in particular those operating by actively locking the modes of a fiber loop laser. Laser mode locking relies on the Kerr effect which takes place in the optical fiber loop when signal light is injected into the loop at sufficient intensity to give rise to the Kerr effect. In those prior art circuits, operation is polarization-independent since prior art circuits are made using an optical fiber that is not polarization dispersive; the effects of polarization are thus averaged out and they cancel. In contrast, those embodiments are highly sensitive to their environment (temperature, vibration, humidity, etc.), thereby restricting practical application thereof.
All-optical clock recovery by active locking of a fiber loop laser is described, for example, in document D1=French patent application FR 94/15555 of Dec. 23, 1994, in the name of Alcatel N. V., and entitled "Disposition de regeneration en ligne d'un signal transmis par solitons via la modulation synchrone des solitons a l'aide d'un miroir optique non-lineaire" Apparatus for in-line regeneration of a signal transmitted by solitons, making use of synchronous modulation of the solitons by means of a non-linear optical mirror! corresponding to U.S. Pat. No. 5,757,529. That document D1 (still not published at the priority date of the present application) is, in the opinion of the Applicant, the closest document in the prior art for understanding the contribution of the present invention, and its contents is expressly incorporated in the present application.
The apparatus described in D1 performs in-line soliton regeneration by synchronous modulation of the solitons by using a non-linear optical loop mirror (NOLM) as an optical modulator, the NOLM modulator being controlled by a clock signal recovered from the soliton signal by clock recovery means which, in a particular embodiment, are all-optical means, e.g. by locking the modes of a fiber loop laser. The laser has an input coupler (C3, C7, C10) which may be a 50/50 coupler or else an asymmetrical coupler. In a particular embodiment, the clock recovery system further includes one or more optical amplifiers and a bandpass filter at the output. As in the present invention, the intended application is optical telecommunications over long distances, in particular by means of solitons.
Like document D3 described below, Document D1 teaches the use of optical clock recovery by locking the modes of a fiber ring laser, but in a fiber laser which does not have polarization dispersion, and therefore its operation is independent of the polarization of the input signal.
Other documents are useful for better understanding of the present invention, and are introduced briefly with a summary of their contribution for this purpose. These documents are also expressly incorporated in the present application as descriptions of the prior art:
D2=N. Finlayson et al. (1992), "Switch inversion and polarization sensitivity of the non-linear optical loop mirror (NOLM)", Optics Lett., Vol. 17, No. 2, pp. 112-114, Jan. 15, 1992. That document teaches that a non-linear loop mirror (NOLM) can be organized as a switch which is little if at all sensitive to the polarization of the light signal to be switched while operating under linear conditions, and that the birefringence of the loop can contribute to the instabilities observed in mode-locked ring lasers.
D3=J. K. Lucek and K. Smith (1993), "All-optical signal regenerator", Opt. Lett., Vol. 18, No. 15, pp. 1226-1228, Aug. 1, 1993, expressly incorporated in the present application as a description of the prior art.
The system known from D3 is shown in FIG. 1 and uses a non-linear optical loop mirror (NOLM) for modulating a clock signal at a first wavelength .lambda..sub.h =1.56 .mu.m with the bit train of a signal to be regenerated, said signal being at a second wavelength .lambda..sub.s =1.54 .mu.m. The clock signal modulated in this way thus constitutes the regenerated signal which has thus changed wavelength (1.56 .mu.m on output instead of 1.54 .mu.m on input).
According to the teaching of that document, the clock signal is recovered from the signal injected on the input fiber F1 by locking the modes of a fiber ring laser RL to generate a train of clock pulses at the bit rate of the signal, with jitter being reduced by the laser mode locking. The length of the laser cavity can be adjusted by means of a device FS for mechanically adjusting transit time to be an integer multiple of the space occupied by one bit in the ring. The device FS is controlled by control means (not shown) which act by feedback to maintain the length of the ring constant. The birefringent devices for polarization control PC are adjusted to minimize the effects of polarization in the ring, which effects tend to reduce laser efficiency.
The clock pulse train is extracted from the ring laser by coupler C6, from which it transits via an optical isolator I and a mechanical device for adjusting transit time FS prior to being injected into input 1 of the coupler C1 of the NOLM. The input signal .lambda..sub.s =1.54 .mu.m is injected into the loop L of the NOLM by the input fiber F3 via the coupler C2. The birefringent devices for controlling polarization PC are adjusted to make the NOLM fully reflective for a clock signal present on the input 1 of the coupler C1 (reflecting towards the input 1) and in the absence of an input soliton signal on the coupler C2. In contrast, when a "1" bit of the signal is injected into the NOLM via the coupler C2, it performs switching that allows the clock signal to pass, which signal then leaves the output fiber F2 via the output 2 of the coupler C1, without time jitter.
The NOLM is thus used as a switch controlled by the bits of the signal to be regenerated, serving to switch the clock signal "on" when the signal bit is "1" (the NOLM is then transparent to the clock signal), and "off" when the signal bit is "0" (the NOLM then reflects the clock signal).
The time window of the signal bit constituting the switch control of the NOLM is larger than that of the clock signal to ensure insensitivity to jitter in the signal to be regenerated (see p. 1227, lefthand column, last paragraph of D3). This is accomplished by using relative "slip" between the signal injected via C2 and the clock signal in the co-propagation direction (clockwise in the figure) due to the chromatic dispersion between the two wavelengths used, with this phenomenon being known as "walk-off".
The system proposed by Lucek and Smith is not the most appropriate for recovering the clock from a train of solitons, for several reasons, and in particular because the polarization control devices (ref. PC in FIG. 1 of D3) need to be adjusted to minimize the effects of polarization in the ring which tend to reduce laser efficiency, while said adjustment needs to be monitored since it depends on environmental parameters (temperature, vibration, . . . ).
The operating reliability of such apparatus on site would appear to be far from satisfactory with respect to these problems.
The document already published at the priority date of the present application and considered by the Applicant as being the closest published document in the prior art is D4=K. Smith and J. K Lucek (1992), "All-optical clock recovery using a mode locked laser", Elect. Lett., 28(19), p. 1814, Sep. 10, 1992. That document describes all-optical recovery of a clock from a soliton signal by mode locking an optical fiber ring laser with said soliton signal being injected into the ring. The apparatus of D4 suffers from the same drawbacks as the apparatus of D3, and for the same reasons.
D5=L. E. Adams et al. (1994), "All-optical clock recovery using a mode-locked figure-eight laser with a semiconductor non-linearity", Electron. Lett., Vol. 30, No. 20, pp. 1696-1697, Sep. 29, 1994. That document teaches another embodiment of a mode-locked laser for all-optical clock recovery.
D6=Uchiyama et al. (1995), "Polarization-independent wavelength-conversion using non-linear optical-loop mirror", Elect. Lett., 31 (21), p. 1862, Oct. 12, 1995 describes a system for converting the wavelength of an optical signal of undetermined polarization applied to the control input of a NOLM as described below with reference to FIG. 2. To make the performance of that apparatus independent of the polarization of the optical signal whose wavelength is to be converted, the NOLM is made out of a polarization-maintaining fiber as in D7 below, and the clock signal is injected to the input of the coupler C1 with polarization at 45.degree. relative to the two neutral axes of the NOLM loop.
D7=K. Uchiyama et al. (1992), "Ultrafast polarization-independent all-optical switching using a polarization diversity scheme in the non-linear optical loop mirror (NOLM)", Electron. Lett., Vol. 28, No. 20, pp. 1864-1866, Sep. 24, 1992. That document shows the use of a NOLM as a switch, which switch is made insensitive to the polarization of the light of the signal to be switched. This is achieved by using a polarization-maintaining fiber that is cut and rotated through 90.degree. at the midpoint of the NOLM loop. The principle is shown in FIG. 2.
By way of example, the NOLM loop is constituted by a two-hole PANDA polarization-maintaining fiber. By performing 90.degree. rotation between the axis A1 and the axis A2 at the propagation halfway point, the fast axis in the lefthand portion becomes the slow axis in the righthand portion of the loop of FIG. 2 (and conversely the slow axis in the lefthand portion becomes the fast axis in the righthand portion). The fiber loop (L) is polarization dispersive, i.e. the propagation speed of light within the fiber is different for polarization in alignment with the fast axis and for polarization that is orthogonal to the fast axis of propagation, i.e. that is aligned with the slow axis of the fiber. It is necessary to eliminate the effects of polarization dispersion, which is achieved by using two equivalent lengths of fiber having polarization-maintaining axes A1 and A2 in a mutually orthogonal disposition so as to cancel polarization dispersion over the length of the loop L.
To make the system independent of the polarization of the switched signal, the polarization of the control signal that is injected into the loop L by the coupler C2 on the control input fiber F3 is injected at 45.degree. to the two orthogonal axes A1 and A2. In the same manner as before, the effects of polarization dispersion cancel.
D8=L. E. Adams et al. (1994), "All-optical clock recovery using a mode-locked figure-eight laser with a semiconductor non-linearity", Electron. Lett., Vol. 30, No. 20, pp. 1696-1697, Sep. 29, 1994. That document teaches another embodiment of a mode-locked laser for all-optical clock recovery.
D9=T. Widdowson et al. (1994), "Soliton shepherding: all-optical active soliton control over global distances", Elect. Lett., 30 (12), pp. 990-991. That document teaches the use of a Kerr type optical modulator apparatus for a "shepherding" application to eliminate soliton jitter in very long distance optical telecommunications links. The term "shepherding" designates time guidance of solitons for time-multiplexed very high data rate systems. The modulator of the invention advantageously replaces the 13 km dispersion shift fiber described in that document.